Can I re-enter Thailand on a visa-exempt basis after traveling to Vietnam?

A friend has just asked me this question,

He’s from the uk and has visited Thailand once, before covid.

Aup mate, hope you're good!

Not sure if you can help me or not, I'm trying to figure something out about Thailand visas.

I'm coming to Thailand for NYE and will be staying for 4 days, then planning to go to Vietnam for 5 days and then go back to Thailand for a couple more weeks after Vietnam.

I know I can get a 30 day visa for Thailand but I'm not sure if I'm allowed to enter Thailand then leave Thailand and then re-enter Thailand?

Or do I need a multiple entry visa instead?

It's not very clear online 🤔

I’ve said that voa will do as it’s just like doing a border run or have the rules changed any?

Thanks folks